The instant invention concerns austenitic iron-chromium-nickel alloys which are suitable for manufacturing machine and construction parts required to resist air oxidation and carburization when exposed to temperatures above 800.degree. C and to resist nitriding at temperatures above 400.degree. C.
The common austenitic chromium-nickel steels and cast alloys subjected to air combustion atmospheres, which develop as a result of the combustion of various heating gases with air, are subjected to air oxidation and are not sufficiently heat-resistant.
The heat-resistant steels which are presently utilized for operational temperatures of approximately 800.degree. C may be austenitic chromium-nickel steels with a silicon content of about 2.5% and a chromium content of about 25%, whereby the silicon should support the effect of the chromium. Higher percentages of these alloying elements, namely the chromium and the silicon, have however the disadvantage that excessively high contents of the same lead to embrittlement and must therefore be avoided.
Austenitic chromium-nickel alloys with higher silicon contents, such as about 4%, have formerly obtained importance only as acid-resistant base materials.
Likewise, with regard to carburization at temperatures above 700.degree. C, the prior art austenitic chromium-nickel steels and cast alloys show obvious tendencies for carburization in a caburizing atmosphere.
The extent of carburization generally increases with increasing temperature. The carbon which is diffusing in the steel separates owing to exceeding of the solubility limit as chromium carbides, preferably at the grain boundaries. In this manner, chromium is taken from the matrix of the steel and the oxidation resistance thereof is reduced. The ductility of the steels and alloys is reduced with increasing precipitation of the brittle and hard obvious carbides. At temperature-change stress, to which many high-temperature parts are subjected, cracks develop after a relatively short period of use. These cracks result in an early failure of the construction parts.
These events are qualitatively the same for rolled and forged steels as well as for casting steels.
In order to prevent the premature failure of austenitic chromium-nickel steels and alloys during use in carburizing atmospheres, there were used in the prior art steels and alloys having a higher nickel content. These steels and alloys were derived from two basic types having 25% Cr, 20% Ni and 35% Ni, 20% Cr. The chromium content of at least 20% guarantees an oxidation resistance which is sufficient for most purposes, while the carburization resistance is greatly improved due to the nickel content of at least 20%.
With respect to nitriding, the common austenitic chromium-nickel steels and casting alloys, at temperatures above approximately 400.degree. C in some nitrogenous atmospheres, for example in an atmosphere of ammonia fission gas, show a noticable tendency for nitriding. Also in the production of melamine, nitrogen is separated which produces nitriding on construction sections. The extent of nitriding generally increases with increasing temperature. A constantly growing nitride layer may develop on the surface; however, there may also be precipitated coarse chromium nitrides at the grain boundaries or inside of the grains, especially at higher temperatures. With an increasingly separated amount of the brittle and hard chromium nitrides, the oxidation resistance of the matrix due to depletion of the chromium and additionally the ductility of the steels are reduced. Most of all, stresses due to changes in temperature will result in the development of cracks and the failure of the construction parts after a relatively short time of operation.
In order to prevent the premature failure of austenitic chromium-nickel steels in nitriding atmospheres, steels with a higher nickel content, especially of the two basic types having 24% Cr, 20% Ni, and 35% Ni, 20% Cr, were utilized. The steels had a silicon content of up to about 2.5%.